NATURAL HISTORY PROJECT-ABSTRACT/PROJECT SUMMARY Congenital disorders of glycosylation (CDG), a rapidly growing group of metabolic diseases characterized by an abnormal glycosylation of biomolecules, were first described in 1980, but the first gene mutation underlying CDG was only found in 1997. Exomic and genomic sequencing engendered a rapid discovery phase of more than 130 different CDG types. Clinical manifestations are variable within and among different types, so physicians from every specialty will likely encounter patients affected by glycosylation defects. Still, limited natural history information is available for the majority of CDG due to their novelty and individual rarity. Existing studies, mostly retrospective, are subject to selection bias, missing data-points and validated patient-reported outcomes. These factors result in knowledge gaps and clinical challenges in CDG treatments and clinical trial readiness. This project's purpose is to perform a multidisciplinary long-term follow-up of a large group of individuals with CDG to define natural history, validate patient reported outcomes and share knowledge on CDG. Our clinical study team has functioned as a virtual expert consortium and will further collaborate by collecting longitudinal data to assess biochemical characteristics, growth, cognitive function, organ system involvement, medical management of symptoms, and patient reported outcomes, as well as morbidity and mortality of individuals with CDGs. To fill in our knowledge gaps in CDG, we will 1) establish the prevalence and severity of specific morbid indicators such as organ system involvement, degree of cognitive disability, and case-fatality associated with various CDG and to establish a dynamic platform to effectively disperse clinically relevant findings to families, non-expert clinicians and researchers, as well as providing a verified method to link these individuals to experts in CDG; and 2) validate the CDG rating scale and patient reported outcome /quality of life measures for CDG. During the follow up we will collect and share samples for biomarker discovery and for bio-banking. The impact of this study will be our increased understanding of CDG and improved care of affected individuals. Natural history and progression studies will further elucidate the pathophysiology of disease and will be essential for clinical trial readiness. Sharing samples and bio-banking will significantly improve diagnostics. Establishing clinical trial outcome measures is essential for successful therapeutic development. This study will additionally enable families and clinicians to access accurate current information empowering them to seek appropriate intervention.